Electronic switches which are suitable for radio frequency (RF) applications and which can be switched between several states of operation by the application of one or more bias voltages to one or more control terminals have widespread applications in such RF devices and components.
For example, modem cellular wireless telephony handsets are generally capable of operating on several different frequency bands and usually require an RF switch to alternately connect a single antenna to the various TX and RX circuit sections of the handset. The RF switch of a cellular handset is often grouped together with RF filters and other RF components in what is commonly referred to as an antenna switching module (ASM) or front end module (FEM). Various applications of RF switches in antenna switching modules are illustrated by Fukamachi et al in US patent application US20040266378A1. It can be seen that for these applications SP2T, SP3T and SP4T RF switches are required. Many other applications for RF switches exist, and the type of switch required is usually governed by parameters specific to the particular application.
An SP2T RF switch includes a common RF port, a first RF input/output port, and a second RF input/output port, the switch has an operation frequency range defined by a lower frequency limit fL and an upper frequency limit fU. An SP2T RF switch furthermore includes two circuit branches where each circuit branch comprises a first end and a second end. The first end of one circuit branch is connected to the first input/output port of the switch and the first end of the other circuit branch is connected to the second input/output port of the switch. The second ends of both circuit branches are connected to the common port of the switch. There are two states of operation of an SP2T RF switch: a first state of operation and a second state of operation. In the first state of operation, a low insertion loss path for RF signals within the operating frequency range of the switch exists between the first input/output port and the common port via one of the circuit branches, and simultaneously there is high isolation between the common port of the switch and the second input/output port for RF signals within the same frequency range; in the second state of operation, a low insertion loss path exists between the second input/output port and the common port via the other circuit branch for RF signals within the operating frequency range of the switch, and simultaneously there is high isolation between the common port of the switch and the first input/output port for RF signals within the same frequency range. Common embodiments of an SP2T RF can furthermore be switched between the first state of operation and the second state of operation actively by the application of a particular combination of control voltages to a number of control terminals of the switch.
A number of prior art embodiments of SP2T RF switches are described below; each prior art embodiment includes a first circuit branch and a second circuit branch where each circuit branch further includes one or more series or parallel active devices, where each active device has two states: an on state where the active devices presents a low impedance path to an RF signal, and an off state where the active devices presents a high impedance path to an RF signal, and where the state of the active device is controlled by the application of a bias voltage to the active device.
In U.S. Pat. No. 3,475,700, Ertel describes several transmit/receive SP2T RF switches which can alternately connect a TX port 14 or an RX port 16 to a common antenna 12. The switch depicted by Ertel in FIG. 1 of U.S. Pat. No. 3,475,700 comprises two series connected PIN diodes 18, 20, each of which can be switched between respective on-states and off-states by the application of a pair of control voltages to control terminals 27, 28. For example, if a negative voltage is applied to control terminal 27, and control terminal 28 is maintained at zero volts, then PIN diode 18 will be in the on-state, and PIN diode 20 will be in the off-state. Thus, TX signals entering the switch at port 14, will be able to pass through the on-state PIN diode 18 directly to the antenna 12, the TX signal will be simultaneously blocked from the RX port 16 by the off-state PIN diode 20. Conversely, if control terminal 27 is maintained at zero volts, and if a negative voltage is applied to control terminal 28, then RX signals entering the switch at the common antenna 12, will be fed directly to the RX port 16, and will be isolated from the TX port 14.
Another embodiment of an SP2T RF switch is depicted by Ertel in FIG. 6 of U.S. Pat. No. 3,475,700; this comprises two parallel connected PIN diodes 166,178, which are switched between respective on-states and off-states by the application of suitable control voltages to control terminals 170, 176, 182. The operation of the SP2T RF switch depicted by Ertel in FIG. 6 of U.S. Pat. No. 3,475,700 is broadly similar to the SP2T RF switch of FIG. 1 of U.S. Pat. No. 3,475,700, except that in the embodiment shown in FIG. 6, the electrical lengths of the pair of microstrip transmission lines between junctions 164 and 158, and between junctions 177 and 158 are both one quarter of a wavelength of the centre frequency of the operating band of the switch. In this way, when one or the other of PIN diodes 166, 178 are in the on-state, the impedance presented at junction 158 by the on-state PIN diode becomes infinitely large, thereby isolating the branch of the circuit including the switched on diode from the antenna 12.
As mentioned above, in each state of operation of an SP2T RF switch, there is a low loss path between the common port of the switch and one of the input/output ports, and simultaneously there is high isolation between the common port of the switch and the other of the input/output ports for RF signals within the operating frequency range of the switch. The principal disadvantage of the various SP2T RF switch embodiments described in U.S. Pat. No. 3,475,700 by Ertel is that the level of isolation offered by each embodiment is limited by the impedance of a single PIN diode in the off-state (FIG. 1) or in the on-state (FIG. 6). Ideally the off-state impedance of a PIN diode is infinite, and the on-state impedance of a PIN diode is zero, this would give rise to infinite isolation for each embodiment, however typical commercially available PIN diodes have an off-state impedance of one or two thousand Ohms, and an on state impedance of one or two Ohms, so that conventional PIN diodes will provide approximately 25 dB of isolation if deployed in the circuits shown in FIG. 1 or FIG. 6 of U.S. Pat. No. 3,475,700.
The isolation of an SP2T PIN diode RF switch can be improved to approximately 40dB if 4 PIN diodes are employed in the switch circuit, two in each circuit branch of the switch. One such SP2T RF switch is described by Kato et al in U.S. Pat. No. 5,519,364. The switch depicted by Kato et al in FIG. 1 of U.S. Pat. No. 5,519,364 is a high isolation SP2T RF switch comprising a pair of shunt PIN diodes in each circuit branch. Another type of SP2T switch architecture is described by Iwata et al, in U.S. Pat. No. 4,220,874. Iwata et al describe a number of embodiments of SP1IT and SP2T RF switches which employ a shunt PIN diode and a series PIN diode in each circuit branch. The SP2T RF switch depicted by Iwata et al in FIG. 4 of U.S. Pat. No. 4,220,874 comprises a pair of shunt PIN diodes D2, D4 and a pair of series PIN diodes D1, D3. The biasing of diodes D1, D2, D3 and D4 is achieved by application of a positive voltage (denoted by V1 in U.S. Pat. No. 4,220,874) or zero volts (denoted by V2 in U.S. Pat. No. 4,220,874) to control terminals S1 and S2 of the switch. The use of two PIN diodes per circuit branch as illustrated in U.S. Pat. No. 5,519,364 and U.S. Pat. No. 4,220,874 offers a substantial increase in the isolation of the switch. FIG. 1 shows a prior art SP2T RF switch according to the embodiment depicted by Iwata et al in FIG. 4 of U.S. Pat. No. 4,220,874.
The SP2T RF switch of FIG. 1 comprises 3 ports: a common port P1, a first input/output port P2, and a second input/output port P3. The switch includes two circuit branches B1, B2, where input/output port P2 is connected to the one end of circuit branch B1, and where input/output port P3 is connected to one end of circuit branch B2, and where the other ends of both circuit branches B1 and B2 are connected to the common port P1. A pair of control voltages applied to control terminals V1 and V2 can set the switch in a first state of operation or a second state of operation according to the logic table given below.
TABLE 1Logic table for prior art SP2T PIN diode switch of FIG. 1.Switch StateV1V2Circuit branch B1Circuit branch B2First State0 V5 VLow Loss betweenHigh Isolationof OperationP1 and P2between P1 and P3Second State5 V0 VHigh IsolationLow Loss betweenof Operationbetween P1 and P2P1 and P3
The switch of FIG. 1 includes PIN diodes D1, D2, D3, D4, where D1 and D2 are the respective shunt and series PIN diodes of circuit branch B1 and where D3 and D4 are the respective shunt and series PIN diodes of circuit branch B2 
The switch further includes DC blocking capacitors C1, C2, C3, C4, C5, C6 which are selected so they have a very low impedance for RF signals within the operating frequency range of the switch. DC biasing components C7 and L3 provide a noise free DC voltage at node M, and DC biasing components C8 and L4 provide a noise free DC voltage at node N. DC biasing component L1 provides a path to ground, via R1, for a DC current arising from a nonzero voltage at node G, and similarly DC biasing component L2 provides a path to ground, via R2, for a DC current arising from a nonzero voltage at node H. Resistor R1 is selected to regulate the current which can flow from node G to ground when a DC voltage is present at node G, and resistor R2 is selected to regulate the current which can flow from node H to ground when a DC voltage is present at node H.
In the first state of operation of the RF switch of FIG. 1, diodes D2 and D3 are forward biased, and diodes D1 and D4 are reverse biased. An RF signal entering circuit branch B1 of the switch at port P2, will be substantially unaffected by reverse biased shunt PIN diode D1 connected to node G, will pass through the forward biased series PIN diode D2, will be isolated from circuit branch B2 by reverse biased series PIN diode D4, and hence will pass without significant attenuation to port P1 of the SP2T RF switch of FIG. 1.
Any small percentage of the RF signal which can pass through reverse biased series PIN diode D4 (due to the finite impedance of the reversed biased PIN diode D4), will have a low resistance path to ground at node H via forward biased shunt PIN diode D3 and capacitor C6 (recall that the value of C6 is chosen to be sufficiently large so that it has a low impedance for RF signals within the operating frequency range of the switch). Hence, the RF signal which enters the switch at P2 will be highly isolated from port P3 of the switch.
Consequently, in the first state of operation of the SP2T RF switch of FIG. 1, an RF signal entering the switch at port P2, will pass without significant attenuation to common port P1 of the switch and will be highly isolated from port P3 of the switch. Similarly, an RF signal entering the switch at common port P1, will pass without significant attenuation to port P2 of the switch, and will be highly isolated from port P3 of the switch.
In the second state of operation of the RF switch of FIG. 1, diodes D1 and D4 are forward biased, and diodes D2 and D3 are reverse biased. An RF signal entering circuit branch B2 of the switch at port P3, will be unaffected by reverse biased shunt PIN diode D3 connected to node H, will pass through the forward biased series PIN diode D4, will be isolated from circuit branch B1 by reverse biased series PIN diode D2, and hence will pass without significant attenuation to port P1.
Any small percentage of the RF signal which can pass through reverse biased series PIN diode D2, will have a low resistance path to ground at node G via forward biased shunt PIN diode D1 and capacitor C5. Hence, the RF signal which enters the switch at P3 will be highly isolated from port P2 of the switch.
Consequently, in the second state of operation of the SP2T RF switch of FIG. 1, an RF signal entering the switch at port P3, will pass without significant attenuation to common port P1 of the switch and will be highly isolated from port P2 of the switch. Similarly, an RF signal entering the switch at common port P1, will pass without significant attenuation to port P3 of the switch, and will be highly isolated from port P2 of the switch.
The SP2T RF switch depicted in FIG. 1 above operates very well within the frequency range of current worldwide cellular systems. However, at very high operating frequencies, such as the frequency band allocated for RF based automotive collision avoidance systems (centered at 24.125 GHz), a number of problems are encountered with the practical implementation of the SP2T RF switch depicted in FIG. 1.
As noted above, in the first state of operation of the SP2T RF switch of FIG. 1, an RF signal entering the switch at port P2 is unaffected by the path of the circuit from node G to ground via reverse biased PIN diode D1 and capacitor C5 because of the high impedance presented by the reverse biased PIN diode D1 connected to node G. This high impedance can be represented by a reflection co-efficient of +1 at node G due to the circuit path containing PIN diode D1 and capacitor C5.
In the second state of operation of the SP2T RF switch of FIG. 1, the high isolation of port P2 from signals entering the switch at port P3 or port P1 is achieved by the combination of the high impedance of reversed biased series PIN diode D2, and the low impedance path to ground at node G through forward biased shunt PIN diode D1 and via capacitor C5. The low impedance path to ground at node G via PIN diode D1 and capacitor C5 can be represented by a reflection co-efficient of −1.
In practical implementations, diode D1 and capacitor C5 will be soldered to a PCB and the PCB will include a first metal track which connects node G to the cathode of PIN diode D1 and a second metal track which connects the anode of diode D1 to capacitor C5.
These metal tracks will have a finite length, and the effect of these metal tracks will be to rotate the phase of the reflection co-efficient at node G due to the path containing PIN diode D1 and capacitor C5 so that it will no longer have the ideal value of +1 in the first state of operation of the RF switch of FIG. 1, or −1 in the second state of operation. The phase rotation caused by the finite lengths of metal tracks which connect node G, PIN diode D1 and capacitor C5 will introduce a substantial loss due to the reverse biased PIN diode D1 in the first state of operation of the SP2T RF switch of FIG. 1 and will substantially reduce the isolation offered by the forward biased PIN diode D3 in the second state of operation of the SP2T RF switch of FIG. 1.
At operating frequencies of 24 GHz, a metal track length of only 1 mm or 2 mm will have a significant effect on the phase of the reflection co-efficient at node G, thereby substantially increasing the loss between ports P1 and P2 and substantially reducing the isolation between ports P1 and P3 in the first operation state of the SP2T RF switch of FIG. 1.
A similar analysis reveals that the effect of the finite lengths of metal tracks required to connect node H, PIN diode D3 and capacitor C6 substantially increases the loss between ports P1 and P3, and substantially reduces the isolation between ports P1 and P2 in the second operation state of the SP2T RF switch of FIG. 1.